1. Field of the Invention
This invention relates to an original reading apparatus for converting the image information of an original to be read into an electrical signal.
2. Description of the Prior Art
Use is made of original reading apparatuses of the type in which an original to be read is illuminated by a tubular illuminating light source such as a fluorescent lamp and is read and scanned while being conveyed by a conveyor roller or the like.
As an example of the reading apparatus of this type according to the prior art, use is made of an apparatus of a structure in which a light source having a length substantially equal to the width of an original to be read conveyed in a horizontal direction is disposed below and near the original to be read, the downward, or vertical, optical axis is changed to a horizontal direction by a mirror disposed below the light source, and the image of the original is formed on an image sensor via a lens disposed in the rear portion of the apparatus.
However, to secure a predetermined length of optical path in such conventional structure, the distance between the mirror and the lens must be made long, and this leads to a problem that the length of the optical system becomes great and the apparatus becomes bulky.
So, there has heretofore been proposed an optical system in which a second mirror for turning back the horizontal optical axis from the mirror below the light source is added and the lens is installed below the light soucre, thereby obtaining a length of optical path about double the length of the apparatus.
In such an optical system, however, the length of the apparatus can be reduced, but the lens of a great diameter must be installed below the light source and the mirror and the total height dimension of these becomes great, which leads to a problem that the height of the apparatus increases.
In a reading apparatus using a lens, efforts to shorten the focal length of the lens have been made to achieve the improved resolving power of the optical system and the compactness of the optical system itself, but there is a limit in making the apparatus compact by such efforts.